The invention relates in the first place to a process for manufacturing a hardenable mixture containing coal ash by mixing the coal ash, optionally a binder and water.
Such a procedure is known from the European Patent Application EP-A-0 222 457 of the present applicant as a phase in the manufacture of granules based on coal ash.
The granules, which are obtained in the manner described in the abovementioned publication, may, after being formed, be hardened at elevated temperature and in the presence of water vapour, as a result of which strong granules are, in general, obtained which can be used for a wide variety of purposes. Prior to the hardening, the granules may, in addition, be moulded into building components which are then subjected to a hardening operation. It has now been found that the procedure described above does not in all cases result in a hardenable mixture from which granules can be formed which, after being hardened, have adequate stability with time. If, for example, an ash is used which contains a certain quantity of free calcium oxide and anhydrous calcium sulphate, it has been found that, under certain circumstances, the granules formed from the mixture begin to exhibit crumbling after the passage of time and even fall apart to form powder.